Mitsubishi L02CPU-P Ethernet not responding

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We have an application using a Mitsubishi L02CPU-P with a built in ethernet module.

At one time the TCP/IP interface became irresponsive and a hard reset was required.

The device is configured for binary communication and works fine using a GOT therminal and also operates fine when queried from a GX Works application.

During debugging of a custom MELSEC application the TCP/IP interface stopped responding and could no longer be located using GX Works and the GOT terminal also lost contact.

The L02CPU-P continued running the control application, but could no longer be contacted. The only way to recover was using the L02CPU-P reset button.

I have heard developers being restrictive using the TCP/IP interface remotely over VPN and unstable connection due to the risk of hanging the device and not being able to recover without a personal visit to the device.

Any experience from L02CPU-P devices requiring hard reset to come "online" again? Or does it enter some default state due to some internal exception handling?

Edited by Flodis

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I am starting to understand at least one MC-protocol or MELSEC "module" hast to be installed/activated on the PLC to enable communication.

When I query the PLC with the magic "Detect Now" button in the GX Works "Ethernet Configuration (Built-in Ethernet Port)"

In the "Host Station" column of the module list there is only one item "Module With No Profile Found" with Protocol UDP.

  • What Ethernet software module should I add to the PLC config for PLC to respond to MELSEC commands?
  • How do I write the added setting back to the PLC without affecting anything else?



Enabling the MELSEC protocol

At lest this forum can be used as a cloud storage for documentation answering my own questions:

Dear Flodis, maybe you can find useful information in the MELSEC-L CPU Module User's Manual (Built-In Ethernet Function) - Page 38?

This is what I learned:

  1. MELSEC protocol and MC-protocol is not he same thing.
    • MELSEC is a proprietary Mitsubishi protocol
    • MC protocol or SLMP is the documented protocol
  2. The protocol is installed as a module in the PLC ethernet config
    • Each module called MC/SLMP has an IP port defined
    • Each port allows one active IP connection at a time
    • The ethernet config allows for a total of 16 modules
  3. It is important to keep a few MELSOFT modules as GX Works may not be able to connect
  4. MELSEC Communication Protocol Reference Manual


In GX Works it may look like this. Notice the checkbox close to Ethernet Conf . button that offers two ways to defines ethernet modules for the protocols to enable for the PLC bult in ethernet module.


In this module configuration view the module of interest is called MC protocol and offers a port number.


In the more advanced ethernet config view the "MC Protocol" module is now presented as "SLMP Connection Module"!!!!!!!

This offers great confusion and takes a while to figure out.

Once the module config is written to the PLC it should respond to MC-protocol requests.

Notice requests can be either ascii or binary and is selected using the checkbox on the ethernet config main screen.

Do not forget to disconnect from the PLC TCP socket when access completed as it offers only single connection per port.




Edited by Flodis
Solution found

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